A variety of methods for controlling an AC rotary machine such as a synchronous machine and an induction machine are known. In a low-speed region only, an AC rotary machine can be driven in a low-cost control system using an inexpensive position sensor whose responsiveness is low in general. Since the output cycle of a Hall sensor itself is slow in a low-speed region, even if a position sensor having a low responsiveness is used, the lowness of responsiveness is not so conspicuous, and thus, such a position sensor can be used for the drive.
Meanwhile, in a case where a position sensor having a low responsiveness is to be used in a high-rotation region, a current cannot be supplied so as to have a desired phase due to the slowness, and the drive characteristics of the AC rotary machine are reduced. If a voltage or a current having a higher frequency than the fundamental frequency of the AC rotary machine is superimposed and the AC rotary machine is controlled based on a result of a position detection using the saliency of the inductance, the AC rotary machine can be driven in a low-speed region without using a position sensor. However, if this method is used to drive the AC rotary machine in a high-rotation region, voltages or currents having a frequency other than the fundamental frequency are generated, which results in disadvantages in terms of the operation efficiency, the voltage utilization rate, and the maximum current.
Therefore, a variety of controllers for an AC rotary machine have been introduced which can smoothly drive the AC rotary machine through sensor-less control, from a low-speed region to a high-speed region.
For example, in the invention disclosed in Patent Literature 1, in order to appropriately generate a phase of a rotational d-q coordinate system required at a basic section of a vector controller without using a magnetic pole position detector, a low-frequency region phase generator for generating a phase for a low-frequency region and a high-frequency region phase generator for generating a phase for a high-frequency region are provided. Then, a phase synthesizer for synthesizing these two types of phases by averaging the phases while weighting the phases by frequency is provided and the synthesized final phase is employed as the phase of the rotational d-q coordinate system.
According to the invention disclosed in Patent Literature 2, in a sensor-less control method for controlling an AC electric motor continuously from zero speed to a high-speed region, a speed estimation value west and a position estimation value θest estimated by a position/speed estimator using a mechanical mathematical expression model are used to perform control such that a position error Δθ becomes zero which is obtained by an exterior product calculation of a first magnetic flux vector calculated by using a rotor angle and a second magnetic flux vector calculated without using the rotor angle.
Further, the invention disclosed in Patent Literature 3 includes an adaptive observer which uses an electric motor model which calculates an angular frequency, an estimated current, and an estimated rotor magnetic flux, based on a current deviation on a rotational two-axis coordinate system (d-q axes), a voltage instruction on the rotational two-axis coordinate system (d-qaxes), and an estimated rotational speed, thereby realizing control of a synchronous electric motor at an especially high rotational speed.
These inventions are considered to be able to smoothly drive an AC rotary machine from a low-speed region to a high-speed region.    Patent Literature 1: Japanese Laid-open Patent Publication No. 10-94298    Patent Literature 2: Japanese Laid-open Patent Publication No. 2006-158046    Patent Literature 3: Republished WO2002/091558